Techniques heretofore in use for collecting dusts generating in factories, when the dusts are products or are to be collected, for example, in order to preserve the working environment, employ a bag filter obtained by knitting glass fibers or fibers made of a heat-resistant synthetic resin to obtain a filter fabric and sewing the fabric into a bag, a felt filter fabric, or a self-supporting filter medium, i.e., a filter element, obtained by sintering a ceramic or synthetic resin powder into a permeable porous object. There also are cases where dusts are collected with an electric dust collector.
Filter elements of that type which have been proposed include one produced by sintering a synthetic resin powder into a self-supporting filter element (see JP-B-Hei. 1 (Sho. 64)-5934) and one produced by sintering a powder of polyethylene, polypropylene, or the like and coating a surface of the sinter with polytetrafluoroethylene particles together with an adhesive (see JP-B-Hei. 2-39926).
However, there are cases where the bag filter constituted of a heat-resistant filter fabric leaks dust particles because the weave texture of the fabric is course. On the other hand, the felt fabric is gradually clogged and comes to have increased air-flow resistance, resulting in the necessity of an increased power for the air blower. It further has a drawback that when the dust collected is removed by back washing, the filter fabric may be damaged by friction with the retainer (filter fabric shape retainer). The dust collection with a ceramic sinter or electric dust collector necessitates a high equipment cost because it is expensive.
Furthermore, the filter element obtained by sintering a synthetic resin powder, when the ambient temperature rises to 70 to 90° C. or higher, begins to change in material and becomes difficult to use as a filter, although it undergoes no change in material and withstands use at around ordinary temperature. The heat resistance may be improved by using a heat-resistant resin such as a polysulfone or the like as a synthetic resin powder. However, such a heat-resistant resin is not always sufficient in resistance to chemicals and mechanical strength, and use thereof results in an increased cost because the material itself is expensive.
A first object of the invention is to provide a low-cost heat-resistant filter element produced from polyethylene particles which is for use in separating and collecting particles from a dust gas, can be continuously used even in a high-temperature environment of about from 70° C. to 130° C., especially from 90° C. to 110° C., is excellent in chemical resistance, and has flexibility and excellent suitability for mass production.
A second object of the invention is to provide processes by which the heat-resistant filter element can be produced at a reduced equipment cost while taking care of the environment.